Homeland Security Secretary Janet Napolitano’s decision to leave the Cabinet this fall means President Barack Obama will have to find a replacement just as deliberations over an immigration overhaul may reach their peak.

Many Republicans have faulted Napolitano and Obama for moving administratively to remove the threat of deportation from over the heads of many young immigrants brought into the country illegally by their parents rather than wait for Congress to pass legislation.

GOP lawmakers have frequently accused DHS of exceeding its legal authority by allowing its immigration enforcement agencies to practice “prosecutorial discretion” that focused apprehension and deportation resources on violent criminals and other high-profile targets instead of those whose offenses were limited to violating immigration law.

Sen. Jeff Sessions, R-Ala., said that policy will define her record. “Secretary Napolitano’s tenure at the Department of Homeland Security was defined by a consistent disrespect for the rule of law,” he said in a written statement. “The resignation of Secretary Napolitano should refocus the attention of Congress on its first task: to ensure that the executive branch faithfully carries out the laws of the land. The most significant obstacle to immigration reform remains President Obama’s selective enforcement of the law.”

Napolitano announced Friday she is resigning as secretary, effective early this fall, to take the job as president of the University of California system.

“For more than four years I have had the privilege of serving President Obama and his Administration as the Secretary of Homeland Security. ... I thank President Obama for the chance to serve our nation during this important chapter in our history, and I know the Department of Homeland Security will continue to perform its important duties with the honor and focus that the American public expects,” she said in her statement.

Obama thanked Napolitano for her service, highlighting her work after the Joplin tornado and Hurricane Sandy, as well as her border security initiatives.

“Since day one, Janet has led my administration’s effort to secure our borders, deploying a historic number of resources, while also taking steps to make our immigration system fairer and more consistent with our values,” he said in a written statement. “And the American people are safer and more secure thanks to Janet’s leadership in protecting our homeland against terrorist attacks. I’ve come to rely on Janet’s judgment and advice, but I’ve also come to value her friendship. And as she begins a new chapter in a remarkable career of public service, I wish her the best of luck.”

Napolitano plans to remain at the department into September, and the administration has no personnel changes in mind until she leaves. The former Arizona governor called the homeland security job the highlight of her career.

“The Department has improved the safety of travelers; implemented smart steps that make our immigration system more fair and focused while deploying record resources to protect our nation’s borders; worked with states to build resiliency and make our nation’s emergency and disaster response capabilities more robust; and partnered with the private sector to improve our cybersecurity.”